Microprocessors are commonly used in control systems to regulate a wide variety of systems from the simple hand held calculator to large mechanical systems such as valves and vehicles. In a specific example, microprocessors are used to control vehicles such as locomotives in order to perform functions including braking, traction control and acceleration. Radio frequency transmitter-receiver pairs are of particular interest for remotely controlling such vehicles.
In a typical remote locomotive control system, the operator communicates with a microprocessor-based controller onboard the locomotive using a remote control device capable of emitting control signals. The operator enters requests into the remote control device via any suitable input such as a keyboard, touch screen or any other suitable system. Typical requests may include brake commands, throttle commands, speed commands or any function that a locomotive may be capable of performing. The remote control device encodes the request into a form suitable for transmission over a pre-determined frequency link. Usually, a tag is added on to the request to indicate the locomotive for which the request is destined as well as an identifier defining the remote control device from which the request originates. The complete request is then modulated at the pre-determined radio frequency and transmitted as a RF signal. Frequencies other than RF can also be used for this purpose.
A receiver aboard the locomotive is equipped with a decoder module that receives and demodulates the RF signal originating from the remote control unit. The signal is then decoded and the validity of the request is verified. Typically, verifying the validity of a request involves performing a sequence of operations to verify if the remote control unit from which the request originates is permitted to issue requests to the particular locomotive as well as verifying if the signal received is intact. Generally, a computer readable medium in the receiver stores an identifier indicative of the remote control unit assigned to the locomotive. The identifier is compared to the tag contained in the received demodulated request. Another operation in the verification of the signal involves verifying if the signal is intact by using a check sum or other suitable error detection or correction algorithm. Verifying that a message is intact is well known in the art of signal processing. If the signal is valid it is then processed further so the command contained in the request can be implemented.
Locomotive control systems of the type described above operate in railroad environments concurrently with many other similar locomotive control devices including remote control units and receivers. Commonly, many remote control modules operate on the same radio frequency channel or on overlapping radio frequency channels often resulting in interference between the various signals. Signals transmitted in overlapping frequency channels cannot be resolved into their respective signals by the receiver module solely on the basis of frequency filtering. The interference of the signals typically causes commands to be lost.
Commonly, existing communication systems make use of a remote control unit designed to repetitively transmit a signal of a fixed length a fixed number of times during a given time segment. In a first example of such a system, a remote control unit sends a command repetitively at a given rate and each remote control unit is assigned a unique repetition rate. The unique repetition rates are selected such that the likelihood of messages interfering with one another is reduced. A variant of this method is described in detail in U.S. Pat. No. 4,245,347 by Hutton et al. whose content is hereby incorporated by reference. In second example, a remote control unit sends a command repetitively to create a succession of signal transmission events where each signal transmission event is spaced in time from a previous signal transmission event by a certain time interval characterized by a certain duration. In such an alternate solution, the durations of time between transmission events are non-uniform. For example the durations of time can be randomly determined or determined according to a pre-determined sequence of time durations. A variant on this method is described in detail in U.S. Pat. No. 6,456,674 by Horst et al. whose content is hereby incorporated by reference.
A deficiency of the systems of the type described above is that a significant amount of valuable bandwidth is occupied by commands being sent repetitively which results into a constraint on the number of locomotive control systems that may adequately operate on a given radio frequency channel.
Consequently there exists a need in the industry to provide an improved system and method for transmitting commands in a control system that alleviates at least in part the deficiencies of prior art systems and methods.